1. Field of the Invention
This invention relates to a C-shaped nail which is inserted into the medullary canal of a fractured along bone in its longitudinal direction, especially to an intramedullary nail which can accelerate bone union through prevention of displacement and angulation of separated fragments as well as minimization of damage to the bone marrow during its insertion by preventing any rotation of the fragments.
2. Description of the Prior Art
Since the femur is a representative long bone, application of the intramedullary nail of this invention to the femur will be principally described in the following. An increased number of bone fracture accidents in elderly persons have been reported in accordance with an increased proportion of elderly persons. Among these accedents, the most frequent ones are proximal fractures of the femur known as femoral trochanteric fracture, femoral neck fracture and the like. Since a prolonged period of confinement to bed may cause internal complications or secondary diseases, it is ideal that load can be applied to the fractured bone at an early stage before bone union is achieved. For such a purpose, it is necessary that separated fragments are brought into contact with each other on their fractured surfaces to restore the appearance of the original bone before the connected fragments are fixed in their positions over a long period so that natural union of the above contact surfaces can be completed. As the means of fixing the fragment there (A) are nail plate methods represented by the AO angle plate method, the Compression hip screw system and the Massie-type sliding nail-plate method, (B) condylocephalic nailing methods (referred to as C-C method hereinafter) represented by the Ender pin method, the K/u/ ntscher nail method and the Harris nail method (described in U.S. Pat. No. 4,135,507), and (C) multiple pinning methods and the like.
The nail plate methods categorized in (A), although exhibiting a relatively good effect in fixing proximal fragments, are insufficiently strong in the fixing of distal fragments and the joint regions between a plate and nails are not free of dynamic weakness. Besides, since it is necessary to develop the fracture site in inserting nails, operation time is prolonged and bone union is delayed due to retarded peripheral blood circulation and breakage of the periosterum. Therefore these methods are disadvantageous to early weight bearing.
The multiple pinning methods categorized in (C) were in use prior to the nail plate methods categorized in (A). These methods are advantageous in that operation is relatively easily performed and operational stress is small. However, in these methods, a very small fixing force is achieved despite insertion of multiple pins and early load bearing is impossible.
From these view points, long nails of the C--C type categorized in (B) have been developed. Such nail has a length almost equal to that of the femur to be treated and is inserted from the distal femoral condyle and passed through the relatively soft medullary canal until its end reaches the proximal femoral head entering the relatively dense cancellous bone located right under the femoral head. Moreover a curved central portion of the nail is brought into contact with the substantia corticalis of the diaphysis. Thus the proximal and the distal fragments are fixed by supporting the inner surface of the substantia corticalis at these points. In the C--C type nail methods, since there is no possibility of directly breaking the substantia corticalis around the fracture site, only a small operational stress is caused. Besides, since a high dynamic intensity in the gravitational direction is realized, load can be applied from a relatively early stage after the operation. However, there is room for improvement since the following problems generally arise: (1) knee disorder due to distal migration, (2) perforation of the femoral head due to proximal migration, (3) varus deformity of the femoral neck due to cutting or the like, (4) external rotation of the distal fragment and (5) fracture of the distal femoral condyle. Degrees and appearances of these problems vary slightly according to the type of nail. For instance, in the Ender pin method in which three thin pins usually are inserted separately while being rotated in the medullary canal, a small space in the medullary canal remains for the latter pin and it can be inserted in an ideal position only with difficulty causing damage to the medullary canal. Besides, since load applied from the proximal side in the longitudinal direction can not be controlled homogeneously, the following troubles may occur: (1) each pin rotates individually causing longitudinal rotation of the distal fragment; (2) part of the pins sink and so protrude from the knee; and (3) part of the pins protrude from the femoral head. Thus clinical problems such as external rotation of the lower limb, varus deformity of the femoral head, knee pain and flexion disturbance of the knee are caused. In contrast, K/u/ ntscher nails and Harris nails are relatively thick and rigid nails having the following characteristics. The Kuntscher nail is inserted through a single surface almost without being turned in the medullary canal thereby causing almost no damage to the medullary canal. However, due to its almost circular cross-section, it has a weak spring force as well as a weak rotation resistance and is thereby relatively unsuitable for the fixation of the proximal fragments. The Harris nail, although considered to exhibit a higher rotation-fixing force than the former two, originally has a shape corresponding to the medullary canal and can be inserted only with difficulty due to its rigidity. In addition, there is a high risk of damage to the medullary canal and it therefore is relatively unsuitable for the fixation of the proximal fragments. The backgrounds of these characteristics will be further described below. The femur has a physiologic anterior curvature, while the main curves of long nails of the C--C type present C-curves as viewed from the lateral surface (L-surface). In other words, the curve of the K/u/ ntscher nail corresponds to the above C-curve. However, it is said to be ideal that the K/u/ ntscher nail be inserted in a straight direction seen from the L-surface of the femur the C-curve of the K/u/ ntscher nail not corresponding to the C-curve of the femur. In such a nail with a secured straightness, its end easily reaches the femoral head thereby exhibiting a good fixing performance. However, since the C-curve of the nail corresponds to the anterior curvature of the femur as mentioned above, the nail tends to obtain a stable attitude by rotating in the medullary canal and the distal fragment rotates due to the rotation of the nail. This is termed external rotation as mentioned before. If such external rotation occurs, a defective reposition is achieved and early load bearing will cause various harmful effects. Here concerning the distal fragments, both the Kuntscher nail and the Harris nail can not be considered to exhibit a sufficient rotation-preventing effect due to the following reasons: (1) the mechanical intensity of the cancellous bone right under the femoral head is weak in a direction perpendicular to the bone trabecula; and (2) the sectional shapes and the sizes of the ends of these nails are an extention of their central portions and these ends have no special rotation-preventing functions. The Harris nail has not only a C-curve corresponding to the above mentioned physiologic anterior curvature but also a distortion corresponding to a physiologic frontward distortion located above the trochanter. That is to say, it has an appearance almost similar to the shape of the medullary canal of the femur. Such an appearance, although seeming to be reasonable when considered only from the situation achieved after the nail is inserted in the medullary canal, is not so when considering the difficulty in inserting such a curved nail into the medullary canal, having anterior curvature and anteversion in the longitudinal direction. Such a three-dimensional nail as the Harris nail having a C-shaped curve and an end distortion perpendicular to the surface of the C-curve, can not be inserted straight without damaging a wide area of the bone marrow. Besides, it can not be twisted and inserted in accordance with the above distortion without disturbing the medullary canal to a substantial degree. Therefore, there is a possibility not only that a physiologically unfavorable effect is caused due to damage to the bone marrow but also that only an insufficient fixing or rotation-preventing effect can be achieved.
In conclusion, the conventional C--C type nail has a purpose only of making its shape similar to the physiologic shape of the femur, and is not sufficiently designed to satisfy both facilitated insertion and a good rotation-preventing effect.